fix: adjust some code structure to satisfy linter

internal/exporter/metrics.go

@@ -1,7 +1,7 @@
 package exporter
 
 import (
-	"log"
+	"log/slog"
 
 	"github.com/prometheus/client_golang/prometheus"
 )
@@ -95,7 +95,7 @@ func (m *Metrics) collectCurrentlyCachedMetric() {
 func (m *Metrics) collectDatabaseTimestampMetric() {
 	timestamp, err := m.exporter.database.Timestamp()
 	if err != nil {
-		log.Printf("failed to read file timestamp: %v", err)
+		slog.Warn("failed to read file timestamp", "error", err)
 		return
 	}